This invention relates in general to the field of non-destructive testing and more particularly to a method for determining the presence, size and location of a crack in an operating (e.g. rotating) shaft. For purposes of this description, a crack is defined as any non-designed physical discontinuity and the term shaft encompasses any axially extending structure which has a length considerably larger than its cross sectional dimension. Such structures take a wide variety of forms and are employed as motor rotors, shafts of pumps, generators, compressors, turbines, etc. Although the present invention is applicable to any such shafts, it will be presented, by way of example, primarily in the context of detecting a crack in an on-line (i.e. operating) reactor coolant pump shaft of a pressurized water reactor (PWR).
Nuclear reactors have been operating and producing useful electricity for many years. Within the last few years, several plants have found cracks in the reactor coolant pump shaft near the thermal barrier.
The large reactor coolant pump of a PWR circulates water out of the reactor vessel into steam generators which in turn pass steam to a steam turbine. The reactor coolant pump system consists of a vertical pump with a vertical motor mounted on the pump from above. In a typical design, the entire shaft system hangs vertically and is supported by a thrust bearing located on the top of the vertical motor. The pump system usually has an overhung impeller and an axial suction inlet from below the pump. The cooling water exits the pump through a single radial discharge in the horizontal direction. A net radial force is developed on the rotating shaft during the operation of the pump. This unidirectional unbalanced force applied to the rotating pump shaft can lead to a fatigue crack in the shaft and subsequent pump shaft failure.
The consequences of an unforeseen pump shaft failure can be dire. A nuclear facility can lose millions of dollars a day in revenues from an unscheduled outage. Further, these pumps are responsible for cooling the reactor, so a failure might lead to a potential melt-down situation and the associated radiation hazard. Since pump shaft replacement is an expensive, time consuming project, it is highly desirable to be able to discover the crack condition early and thus have time to plan and schedule the replacement.
A reliable, early warning method for the on-line identification and quantification of shaft cracks, has not theretofore been available. Existing devices typically collect and analyze vibrational data off a running machine. However, operating vibrational data in the form of 1X (operating speed) and 2X (twice operating speed) amplitude and phase data is usually clouded with electrical, mechanical and background noise such that little useful information relative to the shaft condition can be obtained.
Field studies show that with existing measurement equipment, cracks are not recognizable until they reach a depth of at least 20% of the shaft diameter. The inability to detect a crack at earlier stages, can leave insufficient time to schedule the manpower, parts, etc. required to replace the shaft.
Static test methods which require the shaft under test to be at rest are necessarily limited in their frequency of application and, therefore, in the availability of test results.
A need thus exists for a reliable, on-line shaft crack detection method which can identify the presence, size, and location of a shaft crack in the early stages of crack development and which allows monitoring of crack propagation on a continual basis or whenever desired, during machine operation. The test method has to be applied on site, in a non-destructive fashion, and with minimal radiation exposure to the test personnel. Further complicating the situation is the fact that only limited access to the reactor coolant pump shaft is available.